1. Field of the Invention
The present invention relates to an apparatus for purifying exhaust gas of vehicles. More particularly, the present invention relates to an apparatus for purifying exhaust gas that reduces hydrocarbon (HC), carbon monoxide (CO), particulate material (PM), and nitrogen oxide (NOx) contained in the exhaust gas.
2. Description of Related Art
Generally, exhaust gas flowing out through an exhaust manifold from an engine is driven into a catalytic converter mounted at an exhaust pipe and is purified therein. After that, the noise of the exhaust gas is decreased while passing through a muffler and then the exhaust gas is emitted into the air through a tail pipe. The catalytic converter is a type of diesel particulate filter (DPF) and purifies pollutants contained in the exhaust gas. A catalytic carrier for trapping particulate material (PM) contained in the exhaust gas is in the catalytic converter, and the exhaust gas flowing out from the engine is purified through a chemical reaction therein.
One type of catalytic converter is a selective catalytic reduction (SCR) apparatus. In the selective catalytic reduction (SCR) apparatus, reducing agents such as carbon monoxide and total hydrocarbon (THC) react better with nitrogen oxide than oxygen. Thus, such a catalytic converter is called a selective catalytic reduction apparatus.
According to a conventional apparatus for reducing nitrogen oxide contained in exhaust gas, a diesel oxidation catalyst (DOC) device oxidizes total hydrocarbon (THC) and carbon monoxide (CO), and oxidizes nitrogen monoxide (NO) into nitrogen dioxide (NO2). After that, an SCR device reduces nitrogen oxides (NOx) into nitrogen gas (N2) by using the NO2 generated in the DOC device and a reducing agent. However, as exhaust gas regeneration (ECR) technology and post injection technology is further applied to vehicles, the THC and the CO contained in exhaust gas increases. In addition, the NO2 generated by oxidizing NO is used in the DOC device when the THC and the CO are oxidized during cool starting. That is, the NO2 is used when the THC and the CO are oxidized at a low temperature.
In addition, if the NO2 is generated in the DOC device, the NO2 is used for regenerating soot in a catalyzed particulate filter (CPF) device disposed at the rear of the DOC device. Equation 1 below represents a soot regeneration reaction performed in the CPF device.C+NO2→CO/CO2+NO  Equation 1
Therefore, it is difficult to reduce the NOx in the exhaust gas according to the conventional apparatus for purifying exhaust gas, since the NO2 generated in the DOC device is firstly used in the CPF device and then a part of the remaining NO2 is supplied to the SCR device.
In addition, since, by applying the EGR technology and the post injection technology to vehicles, the increased THC and CO are oxidized only in the DOC device, and the THC and the CO are not fully oxidized.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.